In a general sense, this invention relates to protective coatings applied to metals. More specifically, it relates to metallic coatings which provide oxidation resistance and other attributes to various metal substrates used at high temperatures, e.g., superalloy substrates.
Metal alloys are often used in industrial environments which include extreme operating conditions. For example, the alloys may be exposed to high temperatures, e.g., above about 750xc2x0 C. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating. As an example, gas turbine engines are often subjected to repeated thermal cycling during operation. Furthermore, the standard operating temperature of turbine engines continues to be increased, to achieve improved fuel efficiency.
The turbine engine components (and other industrial parts) are often formed of superalloys, which are usually nickel-, cobalt-, or iron-based. Superalloys can withstand a variety of extreme operating conditions. However, they often must be covered with coatings which protect them from environmental degradation, e.g., the adverse effects of corrosion and oxidation.
Various types of coatings are used to protect superalloys and other types of high-performance metals. One type is based on a material like MCrAlY, where M is iron, nickel, cobalt, or various combinations thereof. These materials can be applied by many techniques, such as high velocity oxy-fuel (HVOF); plasma spray, or electron beam vapor deposition (EB-PVD). Another type of protective coating is an aluminide material, such as nickel-aluminide or platinum-nickel-aluminide. Many techniques can be used to apply these coatings as well. For example, platinum can be electroplated onto the substrate, followed by a diffusion step, which is then followed by an aluminiding step, such as pack aluminiding.
Regardless of coating method, the trend toward higher operating temperatures continues to increase the propensity for corrosion and oxidative attack of the coatings and the underlying metal substrate. Thus, new coating compositions for metal substratesxe2x80x94especially superalloy substratesxe2x80x94would be welcome in the art. The compositions should generally provide better oxidation resistance than currently-used coatingsxe2x80x94especially at use temperatures greater than about 1000xc2x0 C., and preferably, greater than about 1100xc2x0 C. Moreover, the oxidation resistance should generally be maintained when the coated substrate is subjected to a considerable level of thermal cycling, as discussed below.
The new compositions should also be capable of being applied by techniques currently available in the art. Furthermore, the compositions should be based on components that can be varied (in type or amount) to suit specific end uses. For example, the compositions should not require the inclusion of costly components at high levels, for a fairly broad spectrum of applications. Finally, other properties for the new compositions should generally be maintained at acceptable levels, e.g., properties such as corrosion resistance and ductility.
One primary embodiment of the present invention is directed to an oxidation-resistant coating, formed of an alloy comprising:
about 30 to about 55 atom % aluminum; and
about 0.5 atom % to about 3 atom % tantalum;
with the balance comprising at least one base metal selected from the group consisting of nickel, cobalt, iron, and combinations thereof.
In certain preferred embodiments, the alloy also includes a precious metal such as platinum or palladium. Moreover, the alloy often contains chromium. The chromium can be obtained from an underlying substrate by way of diffusion, and/or it can be included as part of the deposited alloy composition. In the same manner, the base metal can diffuse from the substrate, or can be included as part of the deposited alloy.
Some of the embodiments described below also include other elements in the alloy composition. Examples include zirconium, titanium, hafnium, silicon, boron, carbon, yttrium, and combinations thereof. Zirconium is especially preferred for some embodiments. Moreover, other compositions within the scope of this invention advantageously include molybdenum.
As described below, some end use applications for the present invention benefit from a lower level of aluminum, i.e., about 30 atom % to about 45 atom %. Other end use applications utilize a higher level of aluminum, i.e., about 45 atom % to about 55 atom %. In either case, the alloy compositions may include some or all of the other components mentioned above, and further described in this specification.
Another embodiment of this invention is directed to a method for providing environmental protection to a metal-based substrate, such as a superalloy surface. In this method, the alloy composition described above is applied to the substrate, absent any components (e.g., nickel or chromium) which will be incorporated into the composition from the substrate itself. Conventional techniques are used to apply the coating, as described below. Single-stage or multiple-stage processes may be used.
Still another embodiment of this invention is directed to an article, comprising:
(i) a metal-based substrate; and
(ii) an oxidation-resistant coating over the substrate, formed of the alloy outlined above and further described below. In some instances, the oxidation-resistant coating is covered with a thermal barrier coating. The substrate is often a superalloy, and can be a component of a turbine engine.
In this description of the invention, alloy components for the oxidation-resistant coating are advantageously expressed in xe2x80x9catom percentxe2x80x9d. Conversion of these values to xe2x80x9cweight percentxe2x80x9d can easily be carried out, using the atomic weights for each element. As an example for the aluminum/tantalum/base metal composition described above, xe2x80x9cabout 30 to about 55 atom % aluminumxe2x80x9d corresponds to about 15 to about 35.5 weight percent aluminum. The range of xe2x80x9cabout 0.5 atom % to about 3 atom % tantalumxe2x80x9d corresponds to about 2.2 to about 10.3 weight percent tantalum. (The balance is nickel or another base metal, as discussed below). Similarly, for a three-component alloy, using platinum as the exemplary precious metal, the approximate ranges are as follows:
Again, the balance is the base metal.
In the case of an Al/Ta/Cr alloy system, the following conversion-table is helpful (with the balance being the base metal):
Several other range-conversions are provided below, for some of the preferred embodiments of this invention. Other details regarding the various features of this invention are found in the remainder of the specification.